1. Field of the Invention
The present invention relates to the field of optical scanning technology. One aspect the present invention relates to an optical probe optionally for use on a localizer. In another aspect, the present invention relates to a method for scanning an object using the optical probe according to the invention, optionally connected to a localizer.
2. Description of the Related Art
In order to optically measure the position and the dimensions of features in industrial objects such as holes, slots, edges or gap and flush, different solutions presently exist. For example, there are dedicated measuring machines such as profile projectors. However, these machines lack flexibility and measuring speed. There are also machines, generally defined as localizers, capable of positioning and orienting an optical probe in three-dimensional space. The localizer records the position and orientation of the probe with a position-dependent accuracy. These localizers can be, for example, coordinate measuring machines, articulated arms, robots or machining centers. Various optical probes may be mounted on these localizers such as video probes, laser scanners or probes with fringe or pattern projection. All these probes use analogue or digital cameras; most have a charge-coupled device, or CCD-array that is one-dimensional, (i.e. line), or two-dimensional, (i.e. matrix) for receiving the light returning from the object being measured, the light having passed through a series of optics.
One example of a probe of the art is a video probe, which, by back and/or front lighting an object uses image processing to measure a feature of an object. Video probes are basically two-dimensional probes; the third dimension is derived from the auto focus optics, if available. They are quite inaccurate and are not suited for highly curved three-dimensional parts.
Almost all probes that project light and capture all or part of the returning light use triangulation to determine the shape of the object, and to generate a point cloud therefrom. In the triangulation scheme, one or more light sources project light of a certain pattern and in a certain direction onto the object. Viewing optics orientated in certain viewing directions capture the returning light and bring it through one or more optical paths to one or more receivers, such as an analogue or digital camera.
The light projected from an optical probe might originate from one or more ‘projection directions’. For example, where the projected light originates from a discreet projection point, area or volume onto the object, the projected light is referred to as having a single projection direction. Where a probe has two projection directions, the projected light is emitted from two different projection points, areas or volumes in the probe, onto the object. A multiplicity of projection directions requires a corresponding number of projection points, areas or volumes in the probe.
The projected light might form a pattern on the object such as a point, a line, several parallel or intersecting lines, concentric circles, grid of points, etc. Many patterns of light are possible and of different complexity. The pattern might become distorted on the object due to the relief of the object and the angle of the probe relative to the object. The undistorted pattern projected from a projection direction is known as the ‘pattern direction’ and is defined as the pattern direction formed on a flat surface that is orthogonal to the projection direction. Thus, a probe projecting light from two projection direction will have two pattern directions. A multiplicity of pattern directions requires a corresponding number of projection directions in the probe.
A probe having a single point, area or volume therein from which to accept light returning from the object is referred to as having a single ‘viewing direction’. The point, area or volume in practice might be a CCD camera, a single set of receiving lenses, a single set of mirrors. Where the probe accepts light returning from the object from two different directions, it is referred to as having two viewing directions. In practice, a probe having two viewing directions might have two CCD cameras placed slightly apart, or it might have an arrangement of mirrors and/or lenses which direct light from the object from two different points in the probe onto a single CCD camera.
Probes of the prior art which are mounted on a localizer have a single light source, one or two viewing directions, one projection direction and a line, fringes or black-white stripes in the pattern direction for moiré analysis; these probes are inaccurate, and when they are used for measuring features in objects, they are slow and have to be reoriented several times by the localizer in order to measure the features properly. The best accuracy is reached when the lines in the pattern direction are orthogonal to the edges in the object. This rule implies that a hole or circle requires at least two orthogonal sets of stripes in the pattern. The scanning would require a set of translational movements of the probe to cover the complete circle—which would be slow, or a 900 rotational movement of the probe—which would require the user to modify localizers capable of translational movements only by inserting a device capable of movement about one or two rotary axes; such devices are inserted between the localizer head and the probe. These rotary axes may be indexed or continuous. In both cases, translational and/or rotational movements of the probe required to scan a hole or circle are mechanically complicated.
Laser scanners project laser light and have a single pattern direction and one or more viewing directions, usually two CCD-cameras to receive the returning light. With respect to the laser scanner itself, the pattern can be formed by a laser point that is stationary (point laser scanner), or has a linear (line laser scanner) or a circular (circle laser scanner) movement. Other line laser scanners generate a laser line on the object without any moving or rotating parts in the probe itself, mostly by using a cylindrical lens in the laser optics. Most of these scanners are used for digitizing full objects and are relatively accurate. The use of two or more viewing directions in some laser scanners, reduces the chances of line of sight between the receiving optics and the part of the object that is highlighted by the laser becoming blocked by a part of the object itself during scanning. All receiving and emitting optical paths are usually positioned on one line or in one plane.
If a line laser scanner is used for measuring features in objects, ideally the laser line should be orthogonal to the feature itself. This implies that translational movements of the line laser scanner made by the localizer in general are not by themselves sufficient to accurately measure the feature; rotational movements of the scanner would be necessary. For example, a feature with a curved edge will generate translational and rotational movements of the scanner to keep the laser line orthogonal to the edge during measurement. For localizers with translational axes only, this is done by inserting a device capable of one or two rotary movements, either indexed or continuous axes, just before the laser scanner. However, and as already mentioned above, the approach comprising translational and rotational movements of the laser scanner is mechanically complicated.
To avoid the mechanical complicated arrangement, some optical scanners comprise multiple projection directions and only one viewing direction. Alternatively, some have one projection direction that has a suitable pattern (like crossing lines) and multiple viewing directions. Furthermore, some optical scanners of the art project pattern directions simultaneously, and analyze the patterns so-projected as one image. Due the limited depth of view of the optical arrangement in the viewing direction it is difficult to obtain an accurate image of all the projected patterns simultaneously with only one viewing direction. If one projection direction with a complex pattern is used, it is difficult to achieve a good accuracy of feature measurement, even with multiple viewing directions over the complete pattern.
Therefore, there remains a need from the prior art for an easier and improved optical probe for scanning and measuring the features in an object, and methods therefor. The present invention provides an optical probe and methods, which overcome the difficulties and drawbacks of the presently known optical probes and methods.